FIG. 1 is a schematic cross-sectional view of one type of conventional foam molding apparatus, and FIG. 2 is an enlarged schematic cross-sectional view of a portion thereof showing the state when upper and lower halves of a mold for foam molding (which may be referred to below as a “mold”) are closed.
As shown in FIG. 1, a seal portion 14 is provided in the PL portion 12 (the parting line of the product) in the boundary region of upper and lower halves 10a and 10b of a mold body 10 so as to prevent an expanded foam from leaking out from the mold during the foam molding of a urethane resin, for example. In addition, gas which is formed during molding is discharged to the exterior of the mold by minute grooves or holes 16 separately and suitably provided in the wall surface of the upper half of the mold. The seal portion 14 is formed by providing a resin layer on one or both of the wall surfaces of the upper and lower halves of the mold in the upper and lower outer peripheral portions 17, 17 which are peripheral rim portions of the upper and lower halves 10a and 10b. In the figure, the upper half 10a can be closed as shown by dashed lines. The hollow arrow shows the direction of opening and closing.
The conventional minute grooves or holes 16, however, are sometimes easily plugged up at the time of foam molding. In such cases, the pressure inside the mold body increases, and suitable foam molding can not be carried out.
Namely, as shown in the enlarged view of FIG. 2, the seal portion 14 constitutes the PL portion 12 of the foam molding mold 10. If a sudden rise in internal pressure occurs due to the generation of gas accompanying foam molding like that described above, or if the sealing ability of the opposing surfaces of both outer peripheral portions of the upper and lower halves of the mold is inadequate, sudden escape of gas (referred to below as gas release) from the seal portion 14 occurs, and foaming does not take place uniformly. In particular, in the region 26 shown by a hollow circle in the figure, a portion of the molded body is locally hardened (this is referred to as local hardening), and a large number of burrs form along the entire periphery of the mold in the PL portion 12.
Such burrs are ultimately removed in a process referred to as trimming, but the formation of a large number of burrs leads to an increase in costs. Furthermore, in portions of the PL surface where burrs are formed, since the foam density decreases due to protrusion of urethane foam, bubbles become too large, and the hardness decreases below a target value. In addition, even after trimming, the above-described hardened portions remain as a hard layer, and when the molded product is formed into an automobile seat or the like, for example, there are problems such as the product being unpleasant to the touch.
In the past, there have been many attempts at solving such problems. For example, it has been attempted to increase the ability to form a seal between an upper and lower half of a mold. For this purpose, a groove was provided in the outer peripheral portion of the upper half around the entire periphery so as to surround the mold body and to embed a tube of a synthetic resin as a sealing member therein. The tube can function as a type of packing, and a certain degree of sealing effect was obtained. However, in this case, since a complete seal mechanism is now formed, gas collects inside the product, the collected gas forms voids, and the internal pressure increases. Under this high pressure, the sealing properties become inadequate even with the above-described sealing mechanism, and far from burrs being eliminated from a product, the formation of burrs is promoted.